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Mulch is Great for the Ground. Is it Good for the Roof?

Thursday, April 12, 2012

By Kat Harrold

Photo By Jorg Breuning

Mulching season is upon us the air is ripe it's fragrance.  While the good gardener may protect their garden from unwanted weeds with a coat of mulch this can actually kill the functionality of a green roof. 

The fine organic matter produced from the decaying mulch creates a hotbed for weeds that take advantage of the added nutrients.  These same fine particles also clog the filter fabric and drains which result and standing water and roof load issues.

To protect your green roof from unwanted plants there are two main things you can do.  The first is if it is a sedum roof, don't irrigate unless extreme drought lasting more than a month.  The dry conditions often fry the seeds before they get a chance to germinate.  Use only a small amount of slow release fertilizer twice a year for the first 4 years.  Poor soils with a low level of nutrients are ideal for sedums.  Excessive fertilizer can actually be detrimental to sedum growth while simultaneously providing excellent conditions for weed growth.




LL commented on 23-Apr-2012 12:56 PM
What I gather from reading this piece is that mulch is not right for green roofs consisting of sedums, but what about other types of plants that isn’t drought resistant? Ex: A vegetable roof top garden...
Kat Harrold commented on 23-Apr-2012 02:25 PM
While there are many issues with using mulch on a green roof the main one is that when the wood chips break down, the fine organic matter that results from this can clog the filter fabric and drainage system. When this happens you get standing water issues
which then become weight bearing issues for the roof. As far as urban farming is concerned there are a few crop plants like drought hardy herbs and tomatoes that can do well in a green roof system. However, a majority of crops prefer richer more organic soils.
My suggestion if you really want a roof top garden for producing crops would be to have raised planters boxes instead. You can still have a green roof system around or underneath these raised planting beds however it is imperative that the organic soil be
separate from the green roof system.

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